Electrical stimulation of the spinal cord and deep brain has been used for pain relief and to control movement disorders. Electrical leads having many electrodes are implanted in the body such that one or more cathodes and one or more anodes are in optimal locations to produce benefits or to minimize undesirable side effects. FIG. 1 shows a typical implantable electrical stimulation system. An implantable pulse generator 10 generates the electrical signals that will provide the stimulation. A cable 20 connects the implantable pulse generator 10 to a lead 30. Lead 30 contains individual electrodes 31-40. Cable 20 contains ten separate conductors connecting the implantable pulse generator to each of the electrodes 31-40. Implantable electrical stimulation systems are described in co-pending patent application Ser. No. 09/024,162 filed Feb. 17, 1998 and co-pending patent application Ser. No. 08/627,576 filed Apr. 4, 1996. The entire disclosures of both co-pending applications are incorporated herein by reference.
It would be desirable to use a lead with a large number of electrodes, such as sixteen or more, for some therapies. The polarity of each electrode could be assigned and the optimal combinations of cathodes and anodes could be selected for each patient. Another advantage to having several electrodes is that it allows for adjusting the stimulation after the components have been implanted. In particular, an implanted spinal cord stimulation lead can shift up to 0.5 cm or more after being inserted in the body. Ideally, the lead should contain enough electrodes so that some electrodes can be switched off and others switch on after the shift has taken place to avoid the patient undergoing another surgical procedure. It may also be desirable to change the location of the stimulation after the lead has been implanted.
The use of a large number of electrodes on a lead has been limited, in part, because of the limitations imposed by the conductors that connect the implantable pulse generator to the lead. Typical implantable electrical stimulation systems pass up to 20 milliamperes or more of current through each conductor, involving current densities of 10 microcoulombs per square centimeter per phase or more. As a result, each electrode is connected to a sizable conductor in order to minimize energy losses due to impedance and to provide adequate strength to connect the wire to a power supply without substantial risk of breakage. The size of the conductors has made it impractical to connect a large number of conductors between the implantable pulse generator and the electrodes on the lead. Furthermore, it is difficult to obtain the required reliability when using a large number of conductors.
One proposed system places a semiconductor device on the lead to perform a multiplexing operation to minimize the number of conductors connecting the implantable pulse generator to the electrodes. The semiconductor increases the size of the lead and may require significant changes to the standard procedure currently used to implant leads as well as the manufacturing procedures.
Therefore, there exists a need in the art for an implantable electrical stimulation system that includes a large number of electrodes without increasing the size of the lead and that minimizes the number of conductors connecting the implantable pulse generator to the electrodes.